LEARNINGOBJECTIVE: Identify the parts of the circulatory system, and recognize their major components and functions.

The circulatory system, also called the vascular system, consists of blood, heart, and blood vessels. The circulatory system is close circuited (i.e., there is no opening to external environment of the body). The function of this system is to move blood between the cells and the organs of the integumentary, digestive, respiratory, and urinary system that communicate with the external environment of the body. This function is facilitated by the heart pumping blood through blood vessels. The blood travels throughout the body transporting nutrients and wastes, and permitting the exchange of gases (carbon dioxide and oxygen).

BLOOD
Blood is fluid tissue composed of formed elements (i.e., cells) suspended in plasma. It is pumped by the heart through arteries, capillaries, and veins to all parts of the body. Total blood volume of the average adult is 5 to 6 liters.

Plasma
Plasma is the liquid part of blood (fig. 1-31). Plasma constitutes 55 percent of whole blood (plasma and cells). It is a clear, slightly alkaline, straw-colored liquid consisting of about 92 percent water. The remainder is made up mainly of proteins. One of these proteins, fibrinogen, contributes to coagulation.

Blood Cells
The blood cells suspended in the plasma constitute 45 percent of whole blood. Its cells, which are formed mostly in red bone marrow, include red blood cells (RBCs) and white blood cells (WBCs). The blood also contains cellular fragments called blood platelets.

When blood components are separated, the WBCs and platelets form a thin layer, called the buffy coat, between the layers of plasma and RBCs. These layers are illustrated in figure 1-31.

RED BLOOD CELLS.-Red blood cells, or erythrocytes, are small, biconcave, nonnucleated disks, formed in the red bone marrow (fig. 1-32). Blood of the average man contains 5 million red cells per cubic millimeter. Women have fewer red cells, 4.5 million per cubic millimeter. Emotional stress, strenuous exercise, high altitudes, and some diseases may cause an increase in the number of RBCs.

During the development of the red blood cell, a substance called hemoglobin is combined with it. Hemoglobin is the key of the red cell's ability to carry oxygen and carbon dioxide. Thus, the main function of erythrocytes is the transportation of respiratory gases. The red cells deliver oxygen to the body tissues, holding some oxygen in reserve for an emergency. Carbon dioxide is picked up by the same cells and discharged via the lungs.

The color of the red blood cell is determined by the hemoglobin content. Bright red (arterial) blood is due to the combination of oxygen and hemoglobin. Dark

Figure 1-31.-Blood sample illustrating blood components.

Figure 1-32.-A blood smear showing red blood cells, white style="mso-spacerun: yes"> blood cells, and platelets.

red (venous) blood is the result of hemoglobin combining with carbon dioxide.

Red blood cells live only about 100 to 120 days in the body. There are several reasons for their short life span. These delicate cells have to withstand constant knocking around as they are pumped into the arteries by the heart. These cells travel through blood vessels at high speed, bumping into other cells, bouncing off the walls of arteries and veins, and squeezing through narrow passages. They must adjust to continual pressure changes. The spleen is the "graveyard" where old, worn out cells are removed from the blood stream. Fragments of red blood cells are found in the spleen and other body tissues.